@895489045890
I'm not sure if this is what you have happening for you, but if you don't age the sim up on their birthday, it will move to the next day up to 4 times depending on the sim's life stage. By design, there is a four-day grace period at the end of the life stage for teens and above. I have to check, but I believe it is shorter for children and toddlers. At the end of those 4 days, the sim will age up automatically and the sim gets a negative buff for a forgotten birthday. The life bar in the sim panel will show zero days left during those four days. Though, if the birthday on the calendar moves before their birthday, then you've got something else going on.
Hope this helps.